High vacuum apparatus and method



Sept 29 1964 l. ABRAMs ETAL.

HIGH VACUUM APPARATUS AND METHOD Filed June 7, 1962 .Enm Egon. om o Nw n ql m Nm wm mm 1 m m oN (fm AJO- S A Y MAR E SARE N mw m m AHR w n m LJR A m V NmM mwm NT* MDM. vI B mzv mz; oooo. co2 oo. o o m d a 3 s O m /m a #O W' 7 .0. wm o @QW :y

mb- Ul l N @E r o i United States Patent O 3,150,819 IHGH VACUUM APPARATUS AND METHD Irwin L. Abrams, Santa tClara, David l. Hurra, Berkeley,

and llllichael Rivera, Costa Mesa, Calif., assignors to Varian Associates, Palo Alto, Calif., a corporation of Cmifornia Filed June 7, 1962, Ser. No. 200,837 14 Claims. (Cl. 'Z310-69) This invention relates in general to the production of high vacuum and more particularly to an improved method and apparatus for enhancing the pumping elhciency of sputter ion vacuum pumps and systems whereby faster and higher vacua are more readily obtained.

At the present time good high vacuum technique entails the heating of the main vacuum chamber walls so as to produce outgassing thereof. Such bake-out procedures greatly reduce the quantity of adsorbed gases on the internal wall surfaces. In this way a reduction is obtained in the gas load caused by adsorbed gases coming out of the vacuum chamber walls as the system pressure is being lowered.

However, these bake-out procedures as applied to prior art vacuum systems have not been as elfective as might be desired because of certain problems associated with the systems themselves and also with the particular high vacuum pumps utilized therein. For example, in many high vacuum systems the exposed internal surface area presented by the vacuum pump itself forms a significant percentage of the total exposed internal wall area of the entire system. The adsorbed gas coming off the pump walls as pressure is reduced in such a system can result in sizeable gas sources. This problem is not easily alleviated since the form of bake-out typically provided for the main vacuum chamber is not directly applicable to many types of high vacuum pumps. For instance, the mechanical construction and mode of operation for oil diffusion vacuum pumps and the cold traps normally associated with their use are not compatible with high temperature bal e-out.

Some baking of getter ion vacuum pumps of the thermal evaporator type in high vacuum systems has been accomplished by placing heating furnaces around the pumping vessel while pumping on the system with mechanical forepumps. However, here again the eifectiveness of bakeout has been limited because of inconsistencies associated with the heating of the getter ion pump walls. A getter ion pump is most effective when operating with relatively cool (below 100 C.) active gettering surfaces (US. Patent No. 2,850,225). Thus, baking of the pump walls is undertaken only before energization of the getter ion pump at the relatively high pressures associated with mechanical forepurnping. This is a disadvantage since the lower in pressure that outgassing occurs the smaller the quantity of surface gas that will remain on the outgassed walls.

Prior bake-out techniques have also been impractical for the recently developed sputter ion high vacuum pumps. This is because of the massive permanent magnets required for their operation. The extremely large thermal mass presented by these magnets made the attainment of extremely hot (above 300 C.) pump walls for outgassing practically impossible. Also, the magnetic properties of some permanent magnets and the chemical bonding cements used in ferrite magnets can be adversely affected by subjection to elevated temperatures.

The present invention provides a high vacuum sputter ion pump apparatus and operating method therefor which allows the achievement of much faster and higher vacua than was formerly possible. This is accomplished with the use of a novel internal pump heater to provide extremely effective radiant heat to a sputter ion pumps in- 3,150,819 Patented Sept. 29, 1964 rice ternal walls. It has been found that advantageous, relatively low pressure outgassing can be accomplished with this method and apparatus without hindering the performance of the sputter ion vacuum pump. In fact, it has been further shown that the pumping speed of a sputter ion vacuum pump is actually enhanced over certain pressure ranges by operation at high temperatures.

It is therefore the object of the present invention to provide an improved apparatus for the attainment of high vacua.

One feature of this invention is the provision in a sputter ion vacuum pump of an internal heater independent of the gas discharge for outgassing of the pumps internal walls by producing radiant heating thereof.

Another feature of this invention is the provision in a sputter ion pump of an internal radiant heater independent of the gas discharge for enhancing the pumps operating characteristics.

Still another feature of this invention is the use of a sputter ion vacuum pump of the above-featured type in a high vacuum system designed for cyclic operation.

Another feature of this invention is the provision of an extremely efective method of operating a sputter ion vacuum pump of the above-featured types wherein the internal heater is intermittently operated at optimum pressures.

Another feature of this invention is the provision of a control system for automatically operating the abovefeatured apparatus according to the above-featured method.

rl`hese and other features and advantages of the present invention will become more apparent upon a perusal of the following specification taken in connection with the accompanying drawing wherein,

FIG. 1 is a schematic showing of a sputter ion pump vacuum system of the present invention including the novel sputter pump internal radiant heater and control circuit therefor,

FIG. 2 is a diagram in which system pressure in millimeters of mercury versus system pumping time in minutes is plotted for system operation both with internal sputter ion pump heating and without internal sputter ion pump heating, and

FIG. 3 is a fragmentary sectional drawing of another internal radiant heater embodiment of this invention.

Referring to the drawings and to FlG. 1 in particular, there is shown a vacuum system including a vacuum tight chamber lll communicating with a vacuum forepump assembly l2 and also a sputter ion vacuum. pump 13. Suitable tubing il is provided to connect the sputter ion pump 13 to the vacuum chamber il through a vacuum valve assembly 1li while suitable tubing l5 is provided to connect the forepurnp assembly l2 to the sputter pump tubing ld. The forepump tube 16 is fixed to a hand-operated valve 18, opening into a sorption forepump i9 that in this embodiment comprises the vacuum forepump assembly l2. The sputter ion pump 13 has its pumping elements, a cellular anode 2l sandwiched between cathode plates 22, disposed like spokes about a central chamber as taught in U.S. Patent No. 2,983,433. The invention is not limited to this type of sputter ion pump as one skilled in the art can incorporate the novel features herein in other types of sputter ion pumps such as, for example, shown in U.S. Patent No. 2,993,638.

An internal heating assembly formed by a hairpin heater wire 23 provides radiant heating to the internal walls of the sputter ion pump 13. The heater Wire 23 is preferably disposed on the central axis of the sputter ion pump i3 and is supported from a flange cover 25 by and in electrically insulated relationship with a rod 24 having radial arms 26. The heater wire 23 is threaded through insulator eyelets 27 on the ends of radial arms 26. Electrical alessio energy to heat the hairpin wire 23 and to energize the anode electrodes 21 is supplied through suitable electric current feed-throughs 29 which pass through and are insulated from the flange cover 25 and the sputter ion pump casing 2t), respectively. Current is supplied through current feed-throughs 2i? by an external power supply circuit 30 whose operation will be described below.

The vacuum chamber 11 and sputter ion pump tubing 14 are encircled by bake-out heating coils 31 and 33 which receive electrical energy in the form of IR loss from electrical power supplies 32 and 34, respectively.

According to typical prior art vacuum systems and procedures, a vacuum would be produced in the vacuum chamber 11 by first opening valve 13 to provide gas communication between the vacuum chamber and the sorption forepump 19 which had been previously chilled in a standard manner with liquid nitrogen. The vacuum charnber 11 and sputter ion tube 14 would be outgassed by supplying power in the form of heat to external coils 31 and 33. When the pressure in the vacuum chamber 11 had been reduced lto about 10-3 millimeters of mercury the valve 1d would be closed to isolate the sorption forepump 19 from the system and power would be supplied to the sputter ion pump'anode 21. Heating coils 31 and 33 would then be tie-energized allowing the system walls to cool as the sputter ion pump 13 further reduced the system pressure to a desired level. In these prior art vacuum systems the sputter ion pump walls were normally not directly outgassed because of the problems associated with the pump magnets, etc. as mentioned above.

Referring to FIG. 2, curve 36 represents a typical loglog plot of system pressure in millimeters of mercury versus pumping time in minutes for the evacuation process as formerly carried out. As shown, curve 36 indicates the attainment of a 104 mm. Hg vacuum by the sorption pump 19 in about 30 minutes after which the sputter ion pump further reduced the system pressure to l8 mm. Hg after a total elapsed time of about 70 hours.

The internal radiant heater 23 of this invention provides the apparatus whereby an additional step is applied to the evacuation procedure described above to thereby permit the attainment of higher vacua as well as reducing the pumping time required to reach them. In this method, after the sputter ion pump has been energized and the pressure in the system reduced to about 10-5 millimeters of mercury, the heater coil 23 is energized from power supply 3@ to cause heating of the inside wall structure of the sputter ion pump 13. This heating results in rapid increase in the system pressure caused by the thermal outgassing of the internal pump walls. The pressure increase must normally be abated to prevent the system pressure from rising above the operating pressures of the sputter ion pump (above 10`3 millimeters of mercury, for example). Abatement is accomplished in the present invention by de-energizing the heater coil 23 at a pressure of about 105 mm. Hg and then re-energizing it as the pressure is again reduced to about 4X 10-5 mm. Hg. The cycling of the heater coil energization so as to maintain the pressure Within the range between -4 to 14)5 millimeters of mercury is continued until the gas load produced by the outgassing of the pump Walls is insufficient to cause a rise in pressure. Y

This cycling may be accomplished manually or by an automatic switching circuit such as shown in FIG. 2 wherein the positive terminal of a sputter ion pump power supply 40 is connected to the anode electrode feedthroughs 29 and the negative terminal is connected through a current measuring resistor 41 to grounded sputter ion pump casing which is electrically attached to the cathode plates 22. Connected across the current measuring resistor 41 is a voltage amplifier 42 which controls an ori-oit relay 43 having contacts in serieswith an internal heater power supply 44 and the heater coil feedthroughs 29. As the current drawn by the sputter pump 13 through the current measuring resistor 41 is directly proportional to the pressure within the pump casing 20 the relay i3 may be adjusted to respond to the pressure within the vacuum system. Thus, the relay d3 may be adjusted wherein the current drawn by the sputter ion pump 13 at pressures above 10-5 is sufiicient to energize the relay and maintain its normally closed contacts open thereby maintaining de-energization of the heater coil 23. When the pressure in the pump is about lil-5 millimeters of mercury the current drawn by the pump will be insuiicient to maintain the relay 43 energized and its contacts will close to energize the heater coil 23 thereby heating the inner pump walls and causing an increase in pump pressure. As the pressure rises to about 5 104 millimeters of mercury, for example, the current drawn by the pump will again be suiicient to energize the relay 43 and open its contacts thereby again de-energizing the heater coil Z3. This cycling will continue until the gas lead produced by heating the remaining quantity of gas on the sputter ion pump walls is insufficient to cause a pressure rise. The pressure will then continue to fall until a pressure below 10-5 mm. Hg is established after which the manual switch d5 is opened -to permanently de-energize the heater coil 23. The final permanent de-energization of the heater coil 23 is necessary if an ultra high vacuum is to be achieved since the attainment of such a vacuum requires that the ambient temperature within the system be relatively cool so as to minimize outgassing.

The outgassing produced by the internal heater coil 23 greatly reduces the quantity of gas adsorbed on the walis of the sputter pump 13. Thus, after the heater coil 23 has been nally cie-energized the gas load caused by adsorbed gas coming oft" the pump walls will be much smaller allowing the sputter ion pump to not only provide faster pump-down times but also to establish lower base pressures which are dependent upon the quantity of gas adsorbed on the system walls.

The pressure range in which the internal heater coil 23 is used is of considerable importance. The preferred 0perating pressure range is between lO-4 and 105 mm. Hg because of the eiiiciency of the sputter ion pump in this region. Since the throughput (total quantity of gas being pumped) of a sputter ion pump is at a maximum in this pressure region the time required to remove a given quantitty of gas from the system walls will be at a minimum.

lt has also been found that operation of the sputter ion pump is enhanced during the heating cycle provided by the internal heater 23. The increased ambient temperatures within the pump casing 2h cause an improvement in gas conductance into the pumping areas between the sputter ion pump electrodes 21 and 22. This results in an increase of pumping speed for the sputter ion pump.

Referring now to curve 37 in FIG. 2, point a represents the time when the hairpin heater coil Z3 is iirst energized while the procedure before point a is of course the same as that described in connection with the prior art systems above. Curve 37 between points b and c represents the time when power to the internal heater coil 23 was being cycled as described above. Curve .37 then shows the pressure decreasing gradually to point d at which time the power to coil 23 is permanently disconnected and as shown, the pressure then rapidly decreases to obtain a value of 1()-8 millimeters of mercury in about 7 hours. f

As shown, the use of internal heater coil 23 reduced the time required to obtain this particular pressure by about-a factor of l0.

FIG. 3 shows a fragmentary sectional view of another internal sputter pump heater embodiment of this invention. A heater compartment 51 is formed by a hollow tube 52 having a closed inner end (not shown) positioned within the sputter ion pump casing 2t) and an open outer end 53 brazed to a centrally apertured ange cover 5d. inserted into the heater compartment 51 is a heater element 55 having a shoulder portion 56 which is supported by the flange cover 54.

The heating element 55 of this embodiment is used in UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3, 1503319 September 29,I 1964 Irwin L. 'Abrams et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column @Y line en for "155" read 105 Signed and sealed this 16th day of February 1965.,

(SEAL) Attest:

ERNEST W. SWDER i EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A SPUTTER ION VACUUM PUMP APPARATUS COMPRISING A PUMP CASING ADAPTED FOR COMMUNICATION WITH A VACUUM SYSTEM, ANODE AND CATHODE ELECTRODES DISPOSED WITHIN SAID PUMP CASING AND ADAPTED TO SUPPORT A GAS DISCHARGE UPON ENERGIZATION, AND MEANS INDEPENDENT OF SAID GAS DISCHARGE FOR APPLYING HEAT TO THE INTERIOR OF SAID PUMP CASING SO AS TO PRODUCE OUTGASSING THEREOF, SAID MEANS FOR APPLYING HEAT BEING DISPOSED WITHIN SAID PUMP CASING. 